Combine the egg white and powdered sugar in a medium-size mixing bowl and whip with an electric mixer on medium speed until opaque and shiny, about 5 minutes. Add the lemon juice and continue whipping until completely incorporated, about 3 minutes. The lemon juice whitens the royal icing. The royal icing should be light, fluffy, and slightly stiff. You may need to adjust the consistency by adding more egg whites if the icing is too dry or more powdered sugar if it is too wet. To make decorations, fill a cornet half full with royal icing and cut a small opening at the tip. Use the cornet to draw decorations.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
From Dessert Circus, Extraordinary Desserts You Can Make At Home by Jacques Torres